The present invention relates to a process for preparing bisphenol A of high purity, that is, 2,2-bis(4-hydroxyphenyl)propane of high purity.
Bisphenol A is used as a raw material for polycarbonate resins or epoxy resins. Colorless and highly purified bisphenol A is particularly required for polycarbonate resins.
Bisphenol A is prepared by the reaction of acetone with excess phenol in the presence of an acid catalyst or a combination of an acid catalyst and a cocatalyst such as sulfur compounds.
The reaction mixture contains, in addition to bisphenol A, catalyst, unreacted acetone, unreacted phenol, water and other by-products formed by the reaction.
Principal components of the by-products are 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (hereinafter referred to as o,p'-isomer) and 2,2,4-trimethyl-4-(4-hydroxyphenyl)chroman (hereinafter referred to as Dianin's compound). Other components such as trisphenol, polyphenol and unfavorable colored substances are also included. These by-products deteriorate the properties of the resin prepared from bisphenol A.
As the catalyst of the condensation reaction, hydrochloric acid and strongly acidic ion exchange resins have been known to date.
When hydrochloric acid is used as the catalyst, the reaction can proceed at lower temperatures with successive crystallization of adduct of bisphenol A and phenol. In the course of the reaction, the o,p'-isomer in the mother liquor is converted to bisphenol A and hence the amount of the o,p'-isomer is decreased. The reaction with hydrochloric acid catalyst, however, generates a large amount of Dianin's compound as a by-product. As a method for lowering the generation of Dianin's compound, the addition of water has been disclosed by Progil in Japanese Patent Publication, TOKKOSHO 40-7186 (1965). The addition of mercapto compounds has also been reported by Dow Chemical Co. in Japanese Patent Publication, TOKKOSHO 27-5367 (1952). These methods, however, are unfavorable because of complex procedures for the separation and recovery of hydrochloric acid and mercapto compounds as well as the obnoxious odor resulting from mercapto compounds. The generation of Dianin's compound can be reduced by using a greater excess of phenol. On the other hand, the formation of o,p'-isomer increases and a large excess of phenol used must be recovered.
When commonly used ion exchange resins of the strong acid type are employed, by-products are produced in a large amount. Particularly when an ion exchange resin is used for the catalyst, the isomerization due to crystallization of the adduct of bisphenol A and phenol cannot be utilized and thus o,p'-isomer is formed in a much larger amount when compared to the use of hydrochloric acid catalyst.
When the functional groups of the ion exchange resin are modified with mercapto groups such as by reaction with a mercaptoalkylamine, it has been known to markedly decrease the amount of Dianin's compound produced. o,p'-Isomer, however, is still formed as a by-product in a large amount because crystallization of the adduct cannot be utilized.
When an ion exchange resin is used, water formed in the reaction leads to insufficient conversion of acetone. As a result, dehydration of the ion exchange resin is needed in every batchwise reaction. An enormous amount of the resin is also required in a continuous reaction in order to proceed with the conversion of acetone to a significant extent. These results are disclosed by Mitsui Toatsu Chemicals, Inc. in Japanese Laid-Open Patent Publication, TOKKAISHO 61-78741 (1986).
As mentioned above, previously known processes for preparing bisphenol A could not satisfactorily inhibit the formation of two typical impurities, although these processes have their own characteristics, respectively.